Ok im wiring my amplifier and i have a high input feature on my amp that im going to use with my stock hu. To use this high input my instruction booklet says to use the Left back speaker to use for the input. it has 3 wires. L SIGNAL +, L SIGNAL -, AND SIGNAL GROUND. The thing is my back left speaker only has +,- it doesnt have a signal ground. what do i do please help so i can get this working